Delete unused engine_hash.dart script (and test), simplify engine_hash.sh. (#160549)

Closes https://github.com/flutter/flutter/issues/160527.

We don't use the Dart script anyway, and the shell script could be
simplified to the single use in g3 (cl/688973229).

dev/bots/analyze.dart

@@ -2579,6 +2579,7 @@ const Set<String> kExecutableAllowlist = <String>{
   'dev/tools/repackage_gradle_wrapper.sh',
   'dev/tools/bin/engine_hash.sh',
   'dev/tools/format.sh',
+  'dev/tools/test/mock_git.sh',
 
   'packages/flutter_tools/bin/macos_assemble.sh',
   'packages/flutter_tools/bin/tool_backend.sh',